Extend lang items to assert correct target.

This commit extends the existing lang items functionality to assert
that the `#[lang_item]` attribute is only found on the appropriate item
for any given lang item. That is, language items representing traits
must only ever have their corresponding attribute placed on a trait, for
example.

src/librustc/diagnostics.rs

@@ -637,8 +637,8 @@ Erroneous code example:
 ```compile_fail,E0152
 #![feature(lang_items)]
 
-#[lang = "panic_impl"]
-struct Foo; // error: duplicate lang item found: `panic_impl`
+#[lang = "arc"]
+struct Foo; // error: duplicate lang item found: `arc`
 ```
 
 Lang items are already implemented in the standard library. Unless you are
@@ -2116,6 +2116,20 @@ struct Foo;
 ```
 "##,
 
+E0718: r##"
+This error indicates that a `#[lang = ".."]` attribute was placed
+on the wrong type of item.
+
+Examples of erroneous code:
+
+```compile_fail,E0718
+#![feature(lang_items)]
+
+#[lang = "arc"]
+static X: u32 = 42;
+```
+"##,
+
 }
 
 

src/librustc/hir/check_attr.rs

@@ -14,40 +14,80 @@
 //! conflicts between multiple such attributes attached to the same
 //! item.
 
-use syntax_pos::Span;
-use ty::TyCtxt;
-
 use hir;
 use hir::intravisit::{self, Visitor, NestedVisitorMap};
+use ty::TyCtxt;
+use std::fmt::{self, Display};
+use syntax_pos::Span;
 
 #[derive(Copy, Clone, PartialEq)]
-enum Target {
+pub(crate) enum Target {
+    ExternCrate,
+    Use,
+    Static,
+    Const,
     Fn,
+    Closure,
+    Mod,
+    ForeignMod,
+    GlobalAsm,
+    Ty,
+    Existential,
+    Enum,
     Struct,
     Union,
-    Enum,
-    Const,
-    ForeignMod,
+    Trait,
+    TraitAlias,
+    Impl,
     Expression,
     Statement,
-    Closure,
-    Static,
-    Trait,
-    Other,
+}
+
+impl Display for Target {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", match *self {
+            Target::ExternCrate => "extern crate",
+            Target::Use => "use",
+            Target::Static => "static item",
+            Target::Const => "constant item",
+            Target::Fn => "function",
+            Target::Closure => "closure",
+            Target::Mod => "module",
+            Target::ForeignMod => "foreign module",
+            Target::GlobalAsm => "global asm",
+            Target::Ty => "type alias",
+            Target::Existential => "existential type",
+            Target::Enum => "enum",
+            Target::Struct => "struct",
+            Target::Union => "union",
+            Target::Trait => "trait",
+            Target::TraitAlias => "trait alias",
+            Target::Impl => "item",
+            Target::Expression => "expression",
+            Target::Statement => "statement",
+        })
+    }
 }
 
 impl Target {
-    fn from_item(item: &hir::Item) -> Target {
+    pub(crate) fn from_item(item: &hir::Item) -> Target {
         match item.node {
+            hir::ItemKind::ExternCrate(..) => Target::ExternCrate,
+            hir::ItemKind::Use(..) => Target::Use,
+            hir::ItemKind::Static(..) => Target::Static,
+            hir::ItemKind::Const(..) => Target::Const,
             hir::ItemKind::Fn(..) => Target::Fn,
+            hir::ItemKind::Mod(..) => Target::Mod,
+            hir::ItemKind::ForeignMod(..) => Target::ForeignMod,
+            hir::ItemKind::GlobalAsm(..) => Target::GlobalAsm,
+            hir::ItemKind::Ty(..) => Target::Ty,
+            hir::ItemKind::Existential(..) => Target::Existential,
+            hir::ItemKind::Enum(..) => Target::Enum,
             hir::ItemKind::Struct(..) => Target::Struct,
             hir::ItemKind::Union(..) => Target::Union,
-            hir::ItemKind::Enum(..) => Target::Enum,
-            hir::ItemKind::Const(..) => Target::Const,
-            hir::ItemKind::ForeignMod(..) => Target::ForeignMod,
-            hir::ItemKind::Static(..) => Target::Static,
             hir::ItemKind::Trait(..) => Target::Trait,
-            _ => Target::Other,
+            hir::ItemKind::TraitAlias(..) => Target::TraitAlias,
+            hir::ItemKind::Impl(..) => Target::Impl,
         }
     }
 }

src/test/ui/error-codes/E0152.rs

@@ -10,7 +10,7 @@
 
 #![feature(lang_items)]
 
-#[lang = "panic_impl"]
+#[lang = "arc"]
 struct Foo; //~ ERROR E0152
 
 fn main() {

src/test/ui/error-codes/E0152.stderr

@@ -1,10 +1,10 @@
-error[E0152]: duplicate lang item found: `panic_impl`.
+error[E0152]: duplicate lang item found: `arc`.
   --> $DIR/E0152.rs:14:1
    |
 LL | struct Foo; //~ ERROR E0152
    | ^^^^^^^^^^^
    |
-   = note: first defined in crate `std`.
+   = note: first defined in crate `alloc`.
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0718.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(lang_items)]
+
+// Arc is expected to be a struct, so this will error.
+#[lang = "arc"]
+static X: u32 = 42;
+
+fn main() {}

src/test/ui/error-codes/E0718.stderr

@@ -0,0 +1,9 @@
+error[E0718]: `arc` language item must be applied to a struct
+  --> $DIR/E0718.rs:14:1
+   |
+LL | #[lang = "arc"]
+   | ^^^^^^^^^^^^^^^ attribute should be applied to a struct, not a static item
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0718`.

src/test/ui/panic-handler/panic-handler-wrong-location.rs

@@ -0,0 +1,18 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags:-C panic=abort
+
+#![no_std]
+#![no_main]
+
+#[panic_handler]
+#[no_mangle]
+static X: u32 = 42;

src/test/ui/panic-handler/panic-handler-wrong-location.stderr

@@ -0,0 +1,11 @@
+error[E0718]: `panic_impl` language item must be applied to a function
+  --> $DIR/panic-handler-wrong-location.rs:16:1
+   |
+LL | #[panic_handler]
+   | ^^^^^^^^^^^^^^^^ attribute should be applied to a function, not a static item
+
+error: `#[panic_handler]` function required, but not found
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0718`.